Untrained, unsupervised guards with captive enemies devolve into casual cruelty.

Evidence

In the military intelligence unit, where the abuses took place, recruits were cast in the role of guards with no adequate training. They received virtually no supervision while on the job. A ringleader of the group concocted demeaning stunts so that the guards could amuse themselves with detested foes at their mercy. These were the immediate local conditions, but they were only a small part of the story of how intelligence information seeking turned into bizarre abuse. — Albert Bandura, Moral Disengagement, 605